Add PaddleDetection docs & some model configs

* add voc_eval and yolo_darknet_voc

* add yolov3_darknet_voc in MODEL_ZOO

* fix default im_size

* fix MODEL_ZOO note

* fix is_bbox_normalized

* extract map to map_utils.py

* update yolov3_dorknet_voc mixup

* add yolov3_r34_voc

* add yolov3_mobilenet_v1_voc

* fix drop empty in VAL mode

* use cfg.num_classes

* assert metric valid

* enable difficulty can be None

* add comment for bbox_eval

* num_classes in retinanet

* Fix ResizeImage and data transform for SSD
* Fix code style

option should not be overridden by subsequent options of the same module.

* fix gpu detection issue on windows
1. use the fluid.core.get_cuda_device_count() to decide the GPU number
2. ignore the path missing issue

* add log iter argument

* ppdet log time smooth

* ppdet log eta

* refine gan

* fix infer doc and config default

* refine dataset exists

* fix None proc

* add rpn recall function and class-aware rpn

* fix dir and gpu

* fix yolov3 subprocess not exit

* refine print

* fix ptb reader and remove useless language model; test=develop

* add ptb reader; test=develop

* Add usage for distribute Light-NAS searching.

* Refine usage doc of PaddleSlim.